Lyme disease caused by the spirochete Borrelia burgdorferi is the most common vector-borne illness in North America. Coordinated expression of its surface lipoprotein antigens is crucial for its pathogenic strategy. B. burgdorferi abundantly produces outer surface protein A (OspA) in engorged and flat ticks but downregulates OspA and upregulates OspC in response to a bloodmeal. OspC expression ultimately elicits a robust humoral response that poses tremendous pressure on the pathogen. To evade the protective immunity, B. burgdorferi down-regulates ospC. We showed that the downregulation of OspA and OspC in the murine host is achieved via the involvement of newly identified ospC and ospA operators and hypothesize that B. burgdorferi down-regulates ospC and ospA via the interaction of the operators with as-yet unidentified regulatory proteins. This project will first focus on these regulators. In vitro systems will be used to identify these regulators in Aim 1; their essential roles attributed to the ability of B. burgdorferi to evade the immune system and cause persistent infection will be investigated in animal models in Aim 2. The project will be further expanded to explore mechanisms governing the downregulation of OspC. To achieve this goal we have outlined two hypotheses: selection vs. signaling. The molecular basis for the selection hypothesis is that B. burgdorferi generates multiple phenotypes during murine infection, such as spirochetes that abundantly express ospC and others that do not. OspC antibody selectively eliminates phenotypes that express ospC but allows others that do not express ospC to expand. For the signaling hypothesis, B. burgdorferi can sense specific antibody and selectively down-regulate ospC via the induction of the repressor. Aim 3 is proposed to either seek the molecular basis for the selection hypothesis, or to rule out the involvement of a killing process in the antibody-induced ospC down-regulation in the murine model. PUBLIC HEALTH RELEVANCE: Lyme disease caused by the spirochetal bacterium Borrelia burgdorferi, which is transmitted by deer ticks, is the most common vector-borne illness in North America and Europe. Over 20,000 people contract the disease annually in the United States alone. Lyme disease is a multi-system disorder that can result in arthritis, neurological abnormalities, carditis, and cutaneous lesions such as erythema migrans and acrodermatitis chronica atrophicans. If left untreated, the infection and disease may last years. Even worse, up to 10% of Lyme disease patients may develop post-Lyme syndrome, a mysterious illness that can not be cured. Tight regulation of gene expression is crucial for the pathogenic strategy of B. burgdorferi. The goal of the proposed study is to understand how B. burgdorferi regulates gene expression to meet its pathogenic strategy.